Get the ID of the current post's child category

Display every grandchildren on wordpress support forum seems pretty close.

Here is the solution that was proposed:

<ul>
<?php $all = get_pages();   //You could use $args to retrieve posts how you want.
foreach($all as $all) { 
 if($all->post_parent) { 
  if( get_page($all->post_parent)->post_parent ) { 
   $tp = get_page($all->post_parent);
   if( !get_page( $tp->post_parent )->post_parent) { $grandchild_ids[] = $all->ID; }    //catching all the ids of grandchild or secondchild.
  }
 }
}

//Usual args parameter & WP_Query loop
$args = array(
    'post_type' => 'page',
    'post_status' => 'publish',
    'posts_per_page' => 5,
    'orderby' => 'rand',
    'post__in' => $grandchild_ids
  );
$my_query = null;
$my_query = new WP_Query($args);
if( $my_query->have_posts() ) { 
    while ($my_query->have_posts()) : $my_query->the_post(); ?>
        <li><a href="https://wordpress.stackexchange.com/questions/136128/<?php the_permalink(); ?>" title="<?php the_title(); ?>"><?php the_title(); ?></a></li>
    <?php endwhile;
} 
wp_reset_query(); ?>
</ul>

In summary you would get 5 random posts from the list of grandchildren of all the parents.

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